Recently, the trend of the electronic equipments is toward the application with low voltage and high current. The power management technology is developed from the rectification of the rectification diode to the synchronous rectification of the metal-oxide-semiconductor field-effect transistor (MOSFET). The power consumed in the equipment with the rectification diode is more than that consumed in the equipment with the MOSFET; moreover, the performance of the switching power supply can be increased by the latter. The MOSFET is used for the power supply of the LCD TV with low noise and high efficiency and for the power supply of the computer and the telecommunication equipments.
Please refer to FIG. 1(a) showing the circuit of the rectification diode typed LLC series resonant converter in the prior art. The LLC series resonant converter 10 includes a switch circuit 21, a resonant circuit 22, a transformer 23 and a full-wave-rectifier circuit 24.
In FIG. 1(a), the switch circuit 21 is composed of a first switch 211 and a second switch 212, wherein the first switch 211 is composed of a main body Q1 of a first power transistor, a first body-diode DB1 and a first parasitic junction capacitor CO1, and the second switch 212 is composed of a main body Q2 of a second power transistor, a second body-diode DB2 and a second parasitic junction capacitor CO2. The first switch 211 is connected to the second switch 212 to form a half-bridge circuit. In addition, the switch circuit 21 can be a full-bridge circuit. The resonant circuit 22 is composed of a resonant inductor Lr, a resonant capacitor Cr and a magnetizing inductor Lm. It is known by one skilled in the art that the resonant inductor Lr can be composed of a leakage inductor of the transformer 23.
As shown in FIG. 1(a), there is a DC voltage source VDC for the switch circuit 21 to provide an output voltage Vo on a load RL via the resonant circuit 22, the transformer 23, the full-wave-rectifier circuit 24 and a voltage stabilizing capacitor CL. The switch 21 is separated from the resonant circuit 22 and the full-wave-rectifier circuit 24 by the transformer 23 via a primary side winding set Np and series connected secondary side winding sets NS1 and NS2. The full-wave-rectifier circuit 24 is composed of a first rectification diode D1 and a second rectification diode D2 connected to the voltage stabilizing CL. The anode of the first rectification diode D1 is connected to the positive dotted terminal of the secondary side winding set NS1, and the anode of the second rectification diode D2 is connected to the negative dotted terminal of the secondary side winding set NS2. The junction of the secondary side winding sets NS1 and NS2 is used as the ground end of the output voltage Vo, and the cathodes of the rectification diodes D1 and D2 are used as the high voltage end of the output voltage Vo.
Please refer to FIG. 1(b) showing the frequency responses of the resonant converter shown in FIG. 1(a) with various circuit quality factors. As shown in FIG. 1(b), the circuit quality factor QK is related to the load RL and the magnetizing current im, wherein K=Lm/Lr, and the LLC series resonant converter 10 is equivalent to an LC series resonant converter when K approximates to infinity.
As shown in FIG. 1(b), there are a first resonant frequency fr1 and a second resonant frequency fr2. The first resonant frequency fr1 is related to the resonant inductor Lr and the resonant capacitor Cr, the second resonant frequency fr2 is related to the resonant inductor Lr, the magnetizing inductor Lm and the resonant capacitor Cr, and the relationships thereamong are as follows.fr1=1/(2π√{square root over (Lr·Cr)})fr2=1/(2π√{square root over (Lm+Lr)·Cr)})
As shown in FIG. 1(b), there are three operation frequency regions demarcated by the first resonant frequency fr1 and the second resonant frequency fr2. The operation switching frequency fs corresponding to the first frequency region Region-1 has the relationship as fs>fr1. The operation switching frequency fs corresponding to the second frequency region Region-2 has the relationship as fr2<fs<fr1. The operation switching frequency fs corresponding to the third frequency Region-3 has the relationship as fs<fr2. In order to achieve the zero voltage switching and the wide range of voltage stabilization, the first frequency region Region-1 and the second frequency region Region-2 are first considered to be used for the LLC series resonant converter 10; however, in order to achieve the zero voltage switching of the first switch 211 and the second switch 212 and to facilitate the design of the drive circuit, it is well known to use the second frequency region Region-2.
Please refer to FIG. 1(c) showing signals of the resonant converter shown in FIG. 1(a), which operates in the second frequency region. In FIG. 1(c), there are a first switch drive signal VGS1, a second switch drive signal VGS2, a resonant current iL, a magnetizing current im, a power transfer current ip and a periodic change of a resonant capacitor voltage drop VCr operating in the second frequency region Region-2, wherein the power transfer current ip is zero at t0, t1, t2 and t3, there is zero voltage switching on the main body Q1 of the first power transistor and the main body Q2 of the second power transistor at the dead periods of t1-t3 and t4-t6, and the magnetizing current im at the periods is approximately a constant.
Please refer to FIG. 1(d) showing signals of the resonant converter shown in FIG. 1(a), which operates in the first frequency region. In FIG. 1(d), there are a first switch drive signal VGS1, a second switch drive signal VGS2, a resonant current iL, a magnetizing current im, a power transfer current ip and a periodic change of a resonant capacitor voltage drop VCr operating in the first frequency region Region-1, wherein the main body Q1 of the first power transistor and the main body Q2 of the second power transistor are respectively turned off at t1 and t4; thereupon the power transfer current ip is gradually decreased, and the energy is continuously transferred to the load. Therefore, the magnetizing current im is continuously and linearly increased until the main body of the next power transistor is electrified, and the power transfer current ip is zero at t3 and t6. The zero voltage switching on the main boy Q1 of the first power transistor and the main body Q2 of the second power transistor is maintained due to the magnetizing current im with the triangular waveform.
In FIGS. 1(c) and 1(d), the LLC series resonant converter 10 is practiced by using the rectification diode. However, under the identical conditions, the operation of the LLC series resonant converter in the second frequency region Region-2 fails if the LLC series resonant converter uses the MOSFET as the rectification switch.
Please refer to FIG. 2 showing the circuit of the synchronous rectification typed LLC series resonant converter in the prior art. FIG. 1(a) is compared with FIG. 2. The first rectification diode D1 and the second rectification diode D2 shown in FIG. 1(a) are replaced with the third switch 243 and the fourth switch 244, respectively, and the connection of the high voltage end of the output part and the ground end is changed so as to form the circuit shown in FIG. 2. It means that the third switch 243 is composed of a main body Q3 of a third power transistor and a third body-diode DB3, and the fourth switch 244 is composed of the main body Q4 of the fourth power transistor and the fourth body-diode DB4. The main bodies Q3 and Q4 of the power transistors have sources connected to the ground end of the output voltage Vo. The main body Q3 of the third power transistor has a drain connected to the positive dotted terminal of the secondary side winding set NS1. The main body Q4 of the fourth power transistor has a drain connected to the negative dotted terminal of the secondary side winding set NS2. In addition, the fourth switch rectification current iQ4 flows into the negative dotted terminal of the secondary side winding set NS2, and the third switch rectification current iQ3 flows into the positive dotted terminal of the secondary side winding set NS1.
The operation of the synchronous rectification typed LLC series resonant converter 40 in the second frequency region Region-2 is illustrated as follows. It is set that the first switch drive signal VGS1 is identical to the fourth switch drive signal VGS4, the second switch drive signal VGS2 is identical to the third switch drive signal VGS3, and the switch drive signals are identical to those in FIG. 1(c). There is the current through one of the third body-diode DB3 and the fourth body-diode DB4 at the dead periods of t1-t3 and t4-t6, so as to transfer the power from the secondary side to the primary side of the transformer 23, and therefore the circuit cannot operate normally and safely.
When the synchronous rectification typed LLC series resonant converter operates in the first frequency region Region-1, the converter operates normally due to the magnetizing current im with the triangular waveform.
If the conventional synchronous rectification typed LLC series resonant converter 40 shown in FIG. 2 operates in the second frequency region Region-2, there must be different pulse widths for the first switch drive signal VGS1 and the fourth switch drive signal VGS4. Similarly, there must be different pulse widths for the second switch drive signal VGS2 and the third switch drive signal VGS3.
In order to practice the operations in the first frequency region Region-1 and the second frequency region Region-2 for the conventional synchronous rectification drive circuit, the drive circuit is quite complicated. Therefore, the cost is high and the energy is wasted owing to ignoring to increase the conversion efficiency.
Accordingly, the drive circuit of the synchronous rectification typed LLC series resonant converter should be simplified to reduce the volume of the circuit, so as to lower the cost and to achieve high efficiency and low noise.
In order to overcome the disadvantages of the prior art described above, the present invention provides a resonant converter with a synchronous rectification drive circuit.